Memory is typically provided as an integrated circuit(s) formed in and/or on semiconductor die(s), whether alone or in combination with another integrated circuit(s), and is commonly found in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memories have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memories typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the cells, through programming of a charge storage structure, such as floating gates or trapping layers or other physical phenomena, determine the data state of each cell. Common uses for flash memory include personal computers, digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, cellular telephones, and removable memory modules.
Single level memory cells (SLC) can store a single bit of data. Multi-level memory cells (MLC) can store two or more bits of data.
One problem that can occur with programming MLC memory is floating gate-to-floating gate capacitive coupling. The coupling can result in one memory cell disturbing adjacent memory cells, thus causing erroneous data to be stored in the adjacent memory cells.
Multiple step programming algorithms have been used to reduce the floating gate-to-floating gate coupling while also improving threshold voltage distribution widths. One particular multiple step programming operation comprises a prior art touch-up programming operation. This type of programming comprises programming an even page of memory first, reading the even page of memory, programming an odd page of memory then “touching-up” the even page of memory with additional programming pulses. FIGS. 1A-1C illustrate plots of threshold voltage distributions that can result from using a typical prior art multiple step programming operation.
FIG. 1A illustrates threshold voltage distributions after an even page of memory has been programmed. This figure shows an erased state (111) as well as seven programmed states (000-011).
The states of FIG. 1A are each represented by three bits that are the programmed “hard” data of a multiple bit programmed word. The hard data are the actual data, of the multiple bit programmed word, that are used. The programmed word can also comprise “soft” data that are used to indicate a more precise location of the programmed state. For example, the area between each distribution that is indicated by the arrows is the soft data portion of the programmed word that indicates a location of its associated state to the right of the arrow. The soft data might be four bits of the multi-bit programmed word. Thus, the soft data can be considered the least significant bits (LSB) of the programmed word while the hard data can be considered the most significant bits (MSB) of the programmed word.
An even page read operation is performed after the even page has been programmed. Since programming of the memory pages might not be sequential, data stored in a page buffer for programming might be overwritten, after programming, by subsequent data to be programmed to the memory. Thus, during the even page read operation, the even page is read back out into the page buffer so that it can be further programmed during a subsequent touch-up programming operation, as discussed subsequently. This even page read can introduce errors into the programming operation, as subsequently described.
After the even page read is performed, the odd page of the memory cells is programmed. FIG. 1B illustrates the threshold voltage distributions after the odd page of memory has been programmed. The distributions have widened out due to the disturb effects of both program disturb (e.g., multiple programming voltages on the same word line) as well as floating gate-to-floating gate coupling.
It can be seen in FIG. 1B that the overlapping states have the potential to cause errors during reading of the memory since it could be unclear whether the read data belonged in, for example, the 001 state or the adjacent 101 state. In order to tighten up the distributions, an even page touch-up programming operation is performed.
The typical prior art even page touch-up programming operation comprises performing an additional program operation comprising additional programming pulses in order to program in the even page data read during the previous even page read operation. The even page touch-up programming operation programs the memory cells at the lower ends of the distributions to a high threshold voltage such that the memory cells at the lower ends of the distributions are moved up, thus tightening the distributions. FIG. 1C illustrates the distributions after the touch-up programming operation.
A problem with the above-described typical prior art multiple step programming operation is that, since the even page read operation does not use error correction coding, read errors are passed through uncorrected. This uncorrected data is then used during the touch-up programming operation. If the uncorrected data contains errors, the data is re-programmed with the errors during the touch-up operation. This can result in misplacement errors as shown in FIG. 1C by the “tails” 101-107 that are part of each distribution. These tails overlap with an adjacent distribution and represent the hard errors (e.g., error bits that are assigned a low probability of error by an error correction code (ECC) engine) transformed from the original soft errors (e.g., error bits that are assigned a high probability of error by the ECC engine) that can occur when assigning data to the wrong distribution during a read subsequent to the touch-up operation.
For the reasons stated above and for other reasons that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to reduce these programming errors caused by misplacement of data.